1. Statement of the Technical Field
This invention pertains to high-efficiency LED-based backlighting devices used for large screen flat panel LCD displays, in particular for light guide blades having short color mixing distance and uniform distribution of brightness and color in the illumination area.
2. Description of the Related Art
Flat panel LCD displays use backlighting to illuminate the panel for making color images visible. The backlighting is conventionally provided by light sources such as CCFL, but light emitting diodes (LEDs) recently have attracted greater attention and effort. LEDs can be located directly on the back surface of a backlight unit, known as direct backlighting. However, an edge-lit arrangement has advantages such as reducing the depth and making the backlight unit thinner by placing LEDs along the sides and using very thin light guides to uniformly distribute light across the LCD panel. While it is easy to implement LEDs which emit white light in a backlighting system, red, green and blue (RGB) LEDs can provide a much wider color gamut and result in superior color displays. In an edge-lit based backlight unit, light rays from small RGB LED light sources need to be coupled into relatively wide light guides. The light guide may also be referred to as a blade or a light blade. The blade has an input surface from which light is coupled into the light guide. The backlight unit may further include a bezel covering the perimeter of the backlight unit.
Light from the LED light sources is coupled into the blade from one or both lateral sides of the blade, the lateral sides being those sides not adjacent to another blade. A transition region known as the color mixing distance is provided between the LEDs and the illumination region of the blade. The color mixing distance allows for mixing of the colors of light from the individual RGB LEDs before the light is extracted out from the blade. It is desirable that the color mixing distance be kept short so that the size of the bezel surrounding the flat panel LCD display can be kept small.
The blades used in backlighting units usually include a substantially transparent bulk plastic material and the light guide transports light along the length of the blade. An example of a bulk material used in injection-molded light blades is acrylic (PMMA) for low cost, lightweight, and less light absorption. The blade will further include micro-structures (i.e., micro-lenses) on the top and/or bottom surfaces to extract light out from the light guide for illuminating the viewing area of the LCD display. In fact, such light blades can also be used as LED illuminators for Solid State Lighting (SSL) applications.